Electronic cigarettes (ECIGs) are among the most important issues in public health today due to their dramatic increase in popularity. However, very little is known about ECIGs, including what factors may make them more likely to be used and abused. Examining factors that influence nicotine yield, delivery, and subjective effects from ECIGs is essential to understanding their abuse liability. Limited pre-clinical research has revealed that nicotine yield may be influenced by the ratio of the two liquid vehicles, propylene glycol (PG) and vegetable glycerin (VG), most commonly found in ECIG solutions. Specifically, higher proportions of PG result in greater nicotine yields. However, the aerosols generated in this research were not produced by actual ECIG users' puffing behaviors, suggesting that the nicotine yields found may not be indicative of actual ECIG users' nicotine exposure. Moreover, the influence of PG:VG ratio on nicotine delivery and subjective effects associated with ECIG use has not been examined previously. This proposal describes a two-part study. The first examines the effect of PG and VG on nicotine delivery and subjective effects using a within-subjects design. Thirty experienced ECIG users will use an ECIG in five conditions that will differ only by the PG:VG ratio: 100:0, 70:30, 50:50, 30:70, and 0:100 (device voltage, heater resistance, liquid nicotine concentration, puff number, and interpuff interval all will be held constant). Plasma nicotine concentration, subjective effects, and puffing behaviors will be recorded in each condition. The second will generate ECIG aerosols using the puffing behavior recorded from ECIG users in the first part of the study and under identical experimental conditions (e.g., PG:VG ratio). The aerosol produced will be captured and assessed for nicotine yield. The primary hypotheses are that higher proportions of PG will result in greater nicotine delivery and subjective effect profiles suggestive of higher abuse liability in ECIG users and produce aerosols with higher nicotine yields. Results from this project will result in further understanding of the factors that influence the abuse liability of ECIGs and could inform regulation of these products.